Embodiments of the invention relate to riserless mud return systems used in the oil production industry. More particularly, embodiments of the invention relate to a novel system and method for riserless mud return using a subsea pump suspended along a rigid mud return line.
Top hole drilling is generally the initial phase of the construction of a subsea well and involves drilling in shallow formations prior to the installation of a subsea blowout preventer. During conventional top hole drilling, a drilling fluid, such as drilling mud or seawater, is pumped from a drilling rig down the borehole to lubricate and cool the drill bit as well as to provide a vehicle for removal of drill cuttings from the borehole. After emerging from the drill bit, the drilling fluid flows up the borehole through the annulus formed by the drill string and the borehole. Because, conventional top hole drilling is normally performed without a subsea riser, the drilling fluid is ejected from the borehole onto the sea floor.
When drilling mud, or some other commercial fluid, is used for top hole drilling, the release of drilling mud in this manner is undesirable for a number of reasons, namely cost and environmental impact. Depending on the size of the project and the depth of the top hole, drilling mud losses during the top hole phase of drilling can be significant. In many regions of the world, there are strict rules governing, even prohibiting, discharges of certain types of drilling fluid. Moreover, even where permitted, such discharges can be harmful to the maritime environment and create considerable visibility problems for remote operated vehicles (ROVs) used to monitor and perform various underwater operations at the well sites.
For these reasons, systems for recycling drilling fluid have been developed. Typical examples of these systems are found in U.S. Pat. No. 6,745,851 and W.O. Patent Application No. 2005/049958, both of which are incorporated herein by reference in their entireties for all purposes. Both disclose systems for recycling drilling fluid, wherein a suction module, or equivalent device, is positioned above the wellhead to convey drilling fluid from the borehole through a pipeline to a pump positioned on the sea floor. The pump, in turn, conveys the drilling fluid through a flexible return line to the drilling rig above for recycling and reuse. The return line is anchored at one end by the pump, while the other end of the return line is connected to equipment located on the drilling rig.
Positioning the pump on the sea floor requires that the pump be designed and manufactured to withstand hydrostatic forces commensurate with the depth of the sea floor. Also, positioning the pump on the sea floor may be undesirable in certain conditions due to the time needed to retrieve the pump in the event that the pump needs maintenance or bad weather occurs
Thus, embodiments of the invention are directed to riserless mud return systems that seek to overcome these and other limitations of the prior art.